Settling foundation

I have been remodeling a 60-year-old wood-frame house in North Carolina.
Built without a foundation, the house sits on cinder blocks and has settled unevenly so now there is a 3" height difference.
The crawl space under the house is 12" high at the front and 30" high at the back. Is it possible to dig and pour
concrete foundation piers under the house to support jacks and beams to slowly level the house?

Yes, however, you need to contact your local building department for depth requirements for foundation, footings, and/or piers. They will require some type of foundation under your structure, even if it's post and beam, which will require footings or piers.

Have you determined why the house is settling? Do the cinder blocks have a footing beneath them, or are you having problems with water? In a situation like this, I recommend that you bring in a foundation contractor who can raise the home or move it to a different location on the property (if possible) while a new foundation is constructed. As you explore your options, remember that if undisturbed soil cannot be reached at a reasonable depth, a geotech report will be required to determine what is needed to provide adequate support for the footings to support the structure.